The present invention relates to an x-ray generating apparatus using plasma generated by irradiating a laser beam to a target, and to an x-ray microscope comprising an X-ray generating apparatus using such laser plasma.
For example, an x-ray tube or a plasma x-ray source is known as the x-ray source of an x-ray microscope, x-ray laser, an x-ray lithography apparatus, an x-ray photoelectron microscope, an x-ray analyzer or the like.
A plasma x-ray source is arranged to use x-rays generated by interaction between electrons and highly ionized ions in plasma. As a method of generating such high-density plasma, there is known a laser excitation method for example. In this specification, plasma generated by a laser excitation method is called laser plasma. Laser plasma is to be generated by condensing a laser beam on the surface of metal, such as Al, Mo, Au or the like, in the form of pulses each having a width of several ns, the laser beam having been stopped down by a lens or a mirror such that its diameter is for example about 10 .mu.m.about.100 .mu.m.
As an x-ray microscope comprising an x-ray generating apparatus using such laser plasma, there is known a microscope of the type in which x-rays from the x-ray generating apparatus are irradiated to a sample and the transmitted x-rays are measured.
FIG. 21 illustrates a schematic arrangement of a conventional x-ray microscope of the type above-mentioned. This microscope is arranged such that a laser beam 52 is irradiated to a target 51 to generate plasma, that x-rays emitted from the plasma are condensed on a sample 55 in a sample cell 54 by a mirror 53, and that x-rays having passed through the sample 55 are detected by a two-dimensional detector 57 through an enlarging optical system 56.
In an x-ray generating apparatus to be used in such an x-ray microscope or for other purpose, the following arrangement is known as a mechanism for irradiating a laser beam to a target to generate x-rays. That is, flat or disk-like solid metal is for example used as the target, a laser beam is condensed on the surface of the solid metal to generate high-density plasma, and x-rays emitted from free-expanding plasma are guided to the outside of the x-ray generating apparatus. FIG. 22 illustrates, as an example, a schematic arrangement of the x-ray generating apparatus having the arrangement above-mentioned.
In the arrangement in FIG. 22, when a laser beam 7 is focused on and irradiated to the surface of a target 1 of Al, Mo, Au or the like, laser plasma 6 is generated. The laser plasma 6 not only emits scattering particulates composed of neutral particles, charged particles 8 such as ions, electrons and the like, but also x-rays 9. The x-ray generating apparatus is arranged to use, as the x-ray source, such x-rays 9 emitted from the plasma 6. Usually, the x-rays 9 from the plasma 6 are irradiated to an x-ray supply object 11 through an optical element 10 such as a mirror or the like. In an x-ray analyzer for example, the x-ray supply object 11 is used as a sample to be analyzed, x-rays are irradiated thereto and the x-rays on the sample surface are analyzed. In an x-ray microscope, the x-ray supply object 11 is used as a sample to be observed and a detector is disposed therebehind.
In the x-ray generating apparatus having the arrangement above-mentioned, whose pulse shape is controlled by making the laser source the form of multi-pulses or short pulses controls the wavelength of the generated x-rays and the like.
However, such an x-ray generating apparatus of prior art is disadvantageous in that the amount of x-rays to be supplied to the x-ray supply object cannot readily be increased.
More specifically, to improve the x-rays generating efficiency in the apparatus in FIG. 22, it is required to heat or increase the volume of generated plasma by controlling the pulse shape of the laser source. However, since the plasma generally expands freely at a high speed in a vacuum, it is difficult to control the motion of the plasma itself and the plasma momentarily freely expands and spreads. This results in failure to sufficiently improve the x-ray generating efficiency.
In the x-ray generating apparatus in FIG. 22, there is disposed the optical element 10 for introducing the x-rays 9 to the x-ray supply object. In addition to the x-rays 9, scattering particulates composed of charged particles 8 and neutral particles are emitted from the plasma 6, and reach and stick to the surface of the optical element 10, thereby to lower the reflection efficiency of the x-rays 9. This contributes to a reduction in the amount of x-rays to be supplied to the x-ray supply object 11. Thus, the following countermeasure are taken.
To prevent scattering particles from sticking to the optical element 10, there are disposed slits 12 and a scattering particulate preventing means 13 in the direction in which the scattering particles advance from the plasma 6 toward the optical element 10. In the scattering particulate preventing means 13, there may be for example used a method in which there is used a high-speed mechanical shutter arranged such that using a difference in speed between the x-rays and the scattering particulates, the shutter is closed to intercept the passage of the scattering particulates after the high-speed x-rays 9 have passed therethrough. Also, there may be used a method in which a gas inflow device is disposed to let gas to flow from the outside into the path of scattering particulates, causing the gas to come into collision with the scattering particulates to change the tracks thereof. However, such countermeasures cannot securely prevent the scattering particulates from sticking to the optical element 10.
On the other hand, the following apparatus is conventionally known as an x-ray generating apparatus improved in x-ray generating efficiency as compared with the x-ray generating apparatus in FIG. 22. That is, the apparatus is arranged such that an x-ray transmitting film is disposed at one side of the target such that there is formed, between the target and the x-ray transmitting film, a space in which plasma is to be confined. FIG. 23 shows, as an example, a schematic arrangement of such an x-ray generating apparatus.
In the arrangement in FIG. 23, an x-ray transmitting film 72 is so disposed as to form a space 73 adjacent to a tape-like target 71, and plasma generated by irradiating a laser beam 74 to the target 71 is confined in the space 73. In the x-ray generating apparatus in FIG. 23, x-rays are generated in the order as shown in FIG. 24.
When the laser beam 74 is irradiated to the target 71 as shown in FIG. 24(a), a target at the irradiation position is evaporated to generate plasma, and x-rays 75 emitted from the plasma pass through the x-ray transmitting film 72 and are then released to the outside, as shown in FIG. 24(b). At this time, a hole is formed in the target 71 by the laser beam 74. Further, particulates are emitted together with the x-rays 75 from the plasma thus generated, but these particulates are reduced in speed by the x-ray transmitting film 72. As shown in FIG. 24(c), a hole is formed in the x-ray transmitting film 72 by the collision of particulates therewith or by the plasma pressure. Accordingly, the particulates are emitted together with the x-rays 75 through this hole. When the irradiation of a laser beam is finished and the next irradiation is to be conducted, the target 71 and the x-ray transmitting film 72 are moved at their bored portions such that unbored portions of the target 71 and the x-ray transmitting film 72 are located as facing the laser beam irradiation position as shown in FIG. 24(d).
In the x-ray generating apparatus having the arrangement above-mentioned, since the x-ray transmitting film 72 is disposed, the plasma is confined in the space 73 to improve the x-ray generating efficiency. However, the scattering particulates are released through the bored portion of the x-ray transmitting film 72 as above-mentioned. This requires a device for eliminating such scattering particulates as done in the apparatus in FIG. 22. It is therefore required to dispose a scattering particulate preventing means such as a high-speed mechanical shutter 76 or the like as shown in FIG. 23.
The scattering particulate preventing means such as the high-speed mechanical shutter 76 or the like is disposed between the target and a sample (x-ray supply object). Due to the presence of such scattering particulate preventing means, the target-sample distance in the order of cm is required. The x-rays emitted from the target can substantially be regarded as those from a point light source. Accordingly, when the target-sample distance is great, the amount of x-rays irradiated to the sample is disadvantageously reduced.
Further, the requirement for such a scattering particulate preventing means causes the following trouble when such an x-ray generating apparatus is applied to an x-ray microscope shown in FIG. 21.
In the x-ray microscope shown in FIG. 21, the condensing mirror 53 between the target 51 and the sample cell 54 is required because the distance between the sample 55 and the x-ray source is long due to the disposition of a scattering particulate preventing means. Due to the provision of the condensing mirror 53, the wavelength characteristics of x-rays reflected by the condensing mirror 53 should accord with the x-ray wavelength characteristics of the enlarging optical system 56 for enlarging and guiding the transmitted x-rays to the detector side. If the x-ray wavelength characteristics of these two optical systems are not identical with each other, it is not possible to condense the x-rays from the x-ray generating apparatus or enlarge the transmitted x-rays. This fails to produce a good x-ray image to disadvantageously lower the quality of x-ray analysis of the sample.